Various methods are known for producing silicon single crystals to be used for semiconductor substrates, and examples thereof include a Czochralski method (hereinafter called “CZ method”) which is widely used as a rotat inn pulling method.
FIG. 5 is a schematic view of an essential constitution of a pulling apparatus adapted to perform a silicon single crystal growth method based on the CZ method.
The pulling apparatus has an external appearance constituted of a chamber (not shown), and a crucible is disposed at the center of the apparatus. The crucible has a double structure which is constituted of an inner holding vessel (hereinafter simply called “quartz crucible 11a”) made of quartz and having a bottomed cylindrical shape, and an outer holding vessel (hereinafter simply called “graphite crucible 11b”) made of graphite and having a bottomed cylindrical shape adapted to hold an outer surface of the quartz crucible 11a. 
These crucibles are fixed to an upper end of a support shaft 16 to allow the crucibles rotating/elevating movement. Further, a resistance heater 12 is disposed outside crucibles and substantially concentrically with the crucibles, and a silicon raw material of a predetermined weight delivered into the crucible is melted by the heater 12 to form a melt 13.
A pulling wire 15 (or pulling shaft, and these may be called a “pulling member”) configured to rotate about the same axis with the support shaft 16 at a predetermined speed and in a direction which is the same as or opposite to that of the support shaft, is disposed above a central axis of the crucible filled with the melt 13, and a seed crystal 17 is held at a lower end of the pulling member 15.
In such a pulling apparatus, a silicon raw material is delivered into the quartz crucible, and the silicon raw material is melted by the heater disposed around the crucible in an inert gas atmosphere under reduced pressure, followed by immersion of the seed crystal held at the lower end of the pulling member into an upper surface of the formed melt, and followed by elevation of the pulling member while rotating the crucible and the pulling member, to grow a crystal from a lower end surface of the seed crystal.
In the CZ method, in order to eliminate dislocations inherently included in a seed crystal as well as dislocations to be brought about by thermal shock upon contact with the melt, after a necking process for constricting a crystal growing from the lower end surface of the seed crystal into a diameter of about 3 mm, a shoulder to be grown into a body (constant diameter portion) having a predetermined diameter is formed, and a silicon single crystal 14 is subsequently grown at a predetermined diameter. During it, the quartz crucible is rotated in the seed crystal, the same direction or opposite directions. Upon achievement of a targeted length of the single crystal, a constricting operation of a tail of the single crystal is conducted, thereby completing the growth of the single crystal.
As described above, in case of pulling up a silicon single crystal according to the CZ method, the melt obtained by melting the silicon raw material is held in the quartz crucible of the crucible having the double structure. When the quartz crucible holds the silicon melt, the quartz crucible surface is subjected to a high-temperature at 1,500° C. or higher, and the subjected time typically comes up to several tens of hours or longer, though such a time varies depending on conditions such as a filled amount of silicon raw material, a crystal growth rate, and the like.
Further, in order to enhance a single-crystal pulling up productivity, a recharge pulling method (RCCZ method; see “Semiconductor silicon crystal engineering”, pp. 72-73, Fumio Shimura, MARUZEN Co., Ltd., for example) configured to produce a plurality of single crystals from a same crucible has been recently developed. In such a recharge pulling method, a time, during which the quartz crucible is subjected to a silicon melt, may exceed 100 hours.
Typically, brown cristobalite called “brownish ring” is formed at an inner wall surface of the quartz crucible during contact of the quartz crucible with a silicon melt in a high-temperature state, and brownish rings are caused to gradually grow. When such brownish rings are separated from the quartz crucible during a pulling process of the single crystal, the brownish rings obstruct growth of the crystal, to thereby generating dislocations in the crystal.
To prevent such dislocations to be caused by degradation of an inner surface of a crucible, a method for generating devitrification (crystallization) at an inner wall surface of a quartz crucible, by causing the quartz crucible to include one kind or two or more kinds of alkali metal ions of Na, K, and Li, and by applying a DC voltage of a constant value between the quartz crucible and the seed crystal such that an outer wall side of the quartz crucible acts as a positive electrode, and the seed crystal side (silicon single crystal side) acts as a negative electrode, has been disclosed (see JP2006-36568A, for example).
However, even adoption of the above method has sometimes failed to generate a sufficient degree of devitrification at a crucible surface, so that the method is insufficient for prevention of occurrence of dislocations in a silicon single crystal. Further, such a problem that, when a thermal oxidation treatment is conducted for wafers sliced out of the obtained single crystal, extensive variances are caused in thicknesses of oxide films to be formed on surfaces of the wafers, is left.